The present invention relates to x-ray tube technology and is particularly related to a rotating anode x-ray tube having a liquid metal heat pipe apparatus that transfers heat from the region of a focal track of the anode and will be described with particular respect thereto.
Conventional diagnostic use of x-radiation includes the form of radiography, in which a still shadow image of the patient is produced on x-ray film, fluoroscopy, in which a visible real time shadow light image is produced by low intensity x-rays impinging on a fluorescent screen after passing through the patient, and computed tomography (CT) in which complete patient images are digitally constructed from x-rays produced by a high powered x-ray tube rotated about a patient""s body.
Typically, an x-ray tube includes an evacuated envelope made of metal or glass which is supported within an x-ray tube housing. The x-ray tube housing provides electrical connections to the envelope and is filled with a fluid such as oil to aid in cooling components housed within the envelope. The envelope and the x-ray tube housing each include an x-ray transmissive window aligned with one another such that x-rays produced within the envelope may be directed to a patient or subject under examination. In order to produce x-rays, the envelope houses a cathode assembly and an anode assembly. The cathode assembly includes a cathode filament through which a heating current is passed. This current heats the filament sufficiently that a cloud of electrons is emitted, i.e. thermionic emission occurs. A high potential, on the order of 100-200 kV, is applied between the cathode assembly and the anode assembly.
This potential causes the electrons to flow from the cathode assembly to the anode assembly through the evacuated region in the interior of the evacuated envelope. A cathode focusing cup housing the cathode filament focuses the electrons onto a small area or focal spot on a target of anode assembly. The electron beam impinges the target with sufficient energy that x-rays are generated. A portion of the x-rays generated pass through the x-ray transmissive windows of the envelope and x-ray tube housing to a beam limiting device, or collimator, attached to the x-ray tube housing. The beam limiting device regulates the size and shape of the x-ray beam directed toward a patient or subject under examination thereby allowing images to be constructed.
In order to distribute the thermal loading created during the production of x-rays, a rotating anode assembly configuration has been adopted for many applications. In this configuration, the anode assembly is rotated about an axis such that the electron beam focused on a focal spot of the target impinges on a continuously rotating circular path, a focal track, about a peripheral edge of the target. Each portion along the circular path of the focal track becomes heated to a very high temperature during the generation of x-rays and is cooled as it is rotated before returning to be struck again by the electron beam. In many high powered x-ray tube applications such as CT, the generation of x-rays under operating and component design specifications often causes portions of the anode assembly to be heated to a temperature range of 1200-1800xc2x0 C. Temperatures can reach 2500xc2x0 C. at the focal spot in some x-ray tubes. As tube power requirements increase the diameter, mass and rotating velocity are increased. Larger anodes require (i) longer time to reach operational speed of the rotating anode, (ii) decreased x-ray tube and bearing life, (iii) added cost of manufacture and operation and (iv) additional system stresses when the rotating anode x-ray tube is rotated at higher speeds on Computed Tomography gantry systems.
Typically, the anode assembly is mounted to a rotor which is rotated by an induction motor. The anode assembly and rotor are part of a rotating assembly which is supported by a bearing assembly. The bearing assembly provides for a smooth rotation of the anode assembly about its axis with minimal frictional resistance. Bearings disposed in the bearing assembly often consist of a ring of metal balls which surround and rotatably support the rotor to which the anode assembly is mounted. Each of the balls are typically lubricated by application of lead or silver to its outer surface thereby providing support to the rotating assembly with minimal frictional resistance.
As the need for higher power x-ray tubes increases, larger anodes have increased moments of inertia and require more force from the induction motor to accelerate quickly to operational speeds. Some of the disadvantages listed above are interrelated, for example, slower acceleration of the anode induces more heat in the rotor of the x-ray tube. The rotor heat, in addition to the heat transferred from the anode during normal operation, can migrate to the bearings which can result in reduced lubricant efficiency due to evaporation of the lead and silver ball bearing lubricant. Reduced lubricant efficiency is detrimental to tube and bearing life.
As the anode accelerates to operational speed, it passes rotational speeds that create major mechanical resonances in the rotating components of the tube. Less efficient motors, having slower acceleration of the anode to operational speed, increases the amount of time that the anode experiences these major mechanical resonances. This factor also increases mechanical wear of the bearings and has an undesirable effect on tube and bearing life.
During operation in the field it is possible, or in a life critical situation necessary, for the x-ray technician to operate an x-ray tube at operating conditions that result in x-ray tube components experiencing temperatures that exceed operating and component design specifications. In addition to field operation, various processes during manufacture of the tube, such as exhausting and seasoning the tube, also subject an x-ray tube to high thermal loads. Exhausting the tube is the process in which vacuum is drawn in the tube. The tube is operated with internal components at high temperatures while a vacuum pump is operatively attached to the tube. The rate at which gas is removed from the tube and the resulting final pressure of the tube are related to the temperature of the components, such as the anode, during exhaust. The higher the temperature of the component the more effectively the gas is removed from the tube and the lower the pressure of the tube after exhaust.
Seasoning also produces considerable thermal loading for various x-ray tube components. Seasoning is the process in which the tube is exposed to progressively higher voltages and power. This xe2x80x9cburn inxe2x80x9d procedure assists in making the tube more electrically stable at high voltages experienced during tube operation. During the seasoning process the anode target focal track is exposed to some of the highest temperatures that it will experience. During seasoning, the focal track of the anode outgasses and evolves gas molecules into the vacuum envelope, thereby raising the gas pressure.
Damage to x-ray tubes due to thermal loading greater than operating and component design specifications can result in warranty claims and decreased product performance. Therefore, it is desirable to provide an x-ray tube that has a smaller anode with the desired capacity to provide the operating performance for more powerful x-ray applications.
The present invention is directed to an x-ray target that satisfies the need to provide a smaller sized anode with increased operating performance by more effective cooling of the focal track. An apparatus illustrating principles of present invention includes an x-ray tube comprising an evacuated envelope, a cathode assembly located in the evacuated envelope and an anode assembly located in the evacuated envelope in operative relationship with the cathode assembly for generating x-rays. The anode assembly includes, an axis of rotation, a target substrate facing the cathode assembly for generating the x-rays and a back plate located opposite the cathode assembly. At least one heat pipe is located within the anode assembly. The heat pipe comprises a longitudinal cylindrical evacuated shell having a generally central longitudinal axis. A first end of the shell is located in the target substrate and a second end is located in the back plate. A material within the shell is a working fluid for the heat pipe at x-ray tube operating conditions. A porous cylindrical wick within the shell generally extends along the longitudinal axis from the first end of the shell to the second end of the shell. A tubular void within the shell extends along the wick between the wick and shell. A shield attached to the wick along its length. The shield reduces working fluid loss out of the wick into the tubular void between the first and second end during x-ray tube operation.
Another apparatus illustrating principles of the present invention includes an x-ray tube comprising an evacuated envelope, a cathode assembly located in the evacuated envelope, a disk shaped anode assembly located in the evacuated envelope in operative relationship with the cathode assembly for generating x-rays. The anode assembly includes an axis of rotation and a target substrate facing the cathode assembly for generating the x-rays. A heat pipe is located within the anode assembly. The heat pipe comprises an evacuated shell having a first tubular wall and a second tubular wall concentrically spaced apart from one another and defining a void. Each of the first and second tubular walls have a central longitudinal axis that lies generally along the axis of rotation of the anode assembly. The tubular walls are vacuum sealed at a first end of the shell and at a second end of the shell. A material within the shell that is a working fluid for the heat pipe at x-ray tube operating conditions. A porous wick is located within the void of the shell. The wick has a height extending from the first end of the shell to the second end of the shell. A shield is attached to and extends along the height of the wick.
The present invention provides the foregoing and other features hereinafter described and particularly pointed out in the claims. The following description and accompanying drawings set forth certain illustrative embodiments of the invention. It is to be appreciated that different embodiments of the invention may take form in various components and arrangements of components. These described embodiments being indicative of but a few of the various ways in which the principles of the invention may be employed. The drawings are only for the purpose of illustrating a preferred embodiment and are not to be construed as limiting the invention.